1. Field of the Invention
This invention relates to laser transmitters, and more particularly to providing small, high-power laser transmitters that rapidly steer and point over a large field-of-regard (FOR).
2. Description of the Related Art
Laser transmitters traditionally used mechanically controlled mirrors, prisms, or refractive devices to aim, steer and scan laser beams over a FOR. Such mechanically controlled systems have disadvantages in size, weight, power consumption and cost (“SWaP-C”) as well as failure rate and steering limitations.
A non-mechanical approach for beam steering is desirable since it is likely to be smaller, lighter, lower power and less expensive, better SWaP-C, as well as faster, more accurate, more reliable and provide more flexible steering. The challenge is to find a non-mechanical approach that maintains the FOR and range achieved by mechanically steered laser transmitters.
J. L. Ayral et. al. “Phase-conjugate Nd:YAG laser with internal acousto-optic beam steering” OPTICS LETTERS, Vol. 16, No. 16, pp. 1225-1227 Aug. 15, 1991 discloses a Nd:YAG oscillator that delivers a near-diffraction-limited beam that intercepts an acousto-optic deflector. The deflected beam is amplified in a high-gain Nd:YAG zigzag slab amplifier, reflected by an SBS phase-conjugate mirror and amplified again on the second pass. The deflected beam is extracted by reflection on a polarizing beam splitter, after 90° polarization rotation obtained by the double pass through a quarter-wave plate. The main features of this laser source are the following: First, the acoustic-optic deflector is placed on the low-energy beam and consequently does not suffer from any optical damage. Second, owing to the intrinsic properties of phase conjugation, the output beam direction is identical in magnitude to the one imposed by the deflector, independent of any internal reflection that occurs in the laser amplifier. Moreover, thermally induced phase distortions due to the double-pass amplifier are corrected. Third, the laser source is self-aligned.
Jihwan Kim et. al. “Wide-angle, nonmechanical beam steering using thin liquid crystal polarization gratings” Proc. Of SPIE Vol. 7093, 2008 discloses a two-stage technique of beam steering. A fine steering module is constructed of two Optical Phased Arrays (OPAs) designed to cover a ±3.125° range in both the horizontal and vertical directions and expands the steered beam by a factor of 2.5. A subsequent coarse-steering module is based on Liquid Crystal Polarization Grating (LCPGs) that exhibit wide-angle deflection, high overall transmittance, and a very thin package, which thus avoids problems with beam walkoff (loss of clear aperture arising from transverse shifts of the beam within the optical train). The fine and coarse steering modules provide continuous steering over a full 80°×80° FOR.
WO 2014/200581 “Non-Mechanical Beam Steering Tracking System” published Dec. 18, 2014 discloses one or more polarization gratings (PG) coupled to one or more Steerable Electro-Evanescent Optical Refractors (SEEOR) to provide the coarse steering advantage of the PG over a wide and also the continuous fine steering advantage of the SEEOR. Each SEEOR refractor does an excellent job of fine beam control within a narrower FOR (as much as 60°×15°). Vescent Photonics demonstrated a 50°×15° SEEOR. 6 PGs and a single SEEOR can be combined to provide precise fine pointing and a wide (120°×120° or more) FOR. This approach greatly reduces the total number of devices required, when compared to using either technology individually to cover the same FOR. Embodiments of a SEEOR are described in U.S. Pat. Nos. 8,463,080 and 8,311,372. Embodiments of a stack of PGs are described in J. Kim, C. Oh, M. J. Escuti, L. Hosting, and S. A. Serati, “Wide-angle, nonmechanical beam steering using thin liquid crystal polarization gratings,” Advanced Wavefront Control: Methods, Devices, and Applications VI (SPIE, 2008). The terms SEEOR and LCWG are used synonymously in this invention.